This invention relates generally to communications systems and, in particular such communications in a wireless local area network (LAN). Specifically, the invention is directed to assignment of network resources in communication systems using shared multiple access communication media, such as, for instance, frequency hopping patterns, in a multicell radio LAN based on slow frequency hopping spread spectrum signaling.
In a wireless LAN network various configuration parameters or network resources have to be maintained and distributed as base stations and remote stations enter or leave the network. A typical wireless LAN topology is divided into cells. Associated with each cell is a base station connected to a backbone network which acts as an access point and relay for remote stations. To become part of the network, remote stations have to register with one of the base stations. All communications between the remote station and other entities are subsequently handled by the base station with which the remote station has registered. For instance a multicell radio LAN installation based on slow frequency hopping spread spectrum signaling may consist of a set of base stations with overlapping coverage areas. In a frequency hopping (FH) system, the carrier frequency of the transmitter changes at intervals of time, remaining constant between those instants. The period of constant frequency is called a hop and messages may be exchanged during these hops. Efficient methods for controlling and minimizing radio interference between overlapping cells are essential to the reliability and performance of such radio LAN installations. The transmission and reception of messages in a cell of a multicell network of the type that employs identical communication frequencies in different cells, requires control of interference between users. This interference may occur from several sources including transmission from remote stations that lie in overlapping areas between adjacent cells and transmissions from base stations if these overlapping cell areas contain one or more remote users. Assigning different frequency hopping sequences or patterns to base stations with overlapping coverage areas allows to control and limit interferences.
The following U.S. Patents and European Patent applications teach various aspects of mobile communications using wireless transmission media.
U.S. Pat. No. 5,239,673 teaches a scheduling method for efficient frequency reuse in a multi-cell wireless network served by a wired local area network. One method of the invention circulates a high priority token among a plurality of header stations connected to the wired network. Reception of the token causes the receiving header station to perform wireless communications. When finished, the header station forwards the token to another header station.
The following two U.S. Patents show communication systems having overlapping coverage areas: U.S. Pat. No. 4,597,105, Jun. 24, 1986, entitled xe2x80x9cData Communications System having Overlapping Receiver coverage Zonesxe2x80x9d to Freeburg and U.S. Pat. No. 4,881,271 issued Nov. 14, 1989, entitled xe2x80x9cPortable Wireless Communication Systemsxe2x80x9d to Yamauchi et al. provide for a hand-off of a subscriber station from one base station to another by the base station continually monitoring the signal strength of the subscriber station.
The following U.S. Patents teach various aspects of wireless communication networks.
In U.S. Pat. No. 4,792,942, issued Dec. 20, 1988 entitled xe2x80x9cWireless Local Area Network for Use in Neighborhoodsxe2x80x9d S. Mayo describes a local area network that includes transceiver stations serially coupled in a loop.
In U.S. Pat. No. 4,730,310 issued Mar. 8, 1988 entitled xe2x80x9cTerrestrial Communications Systemsxe2x80x9d Acampora et al. describe a communications system that employs spot beams, TDMA and frequency reuse to provide communication between a base station and remote stations.
In U.S. Pat. No. 4,639,914, issued Jan. 27, 1987 entitled xe2x80x9cWireless PBX/LAN System with Optimum Combiningxe2x80x9d Winters discloses a wireless LAN system that employs adaptive signal processing to dynamically reassign a user from one channel to another.
In U.S. Pat. No. 4,926,495, issued May 15, 1990 entitled xe2x80x9cComputer Aided Dispatch Systemxe2x80x9d Comroe et al. disclose a computer aided dispatch system that includes a master file node and a plurality of user nodes. The master file node maintains a record for each subscriber and automatically transmits an updated record to each dispatcher attached to a subgroup in which the subscriber operates.
In U.S. Pat. No. 4,456,793, issued Jun. 26, 1984 W. E. Baker et al. describe a cordless telephone system having infrared wireless links between handsets and transponders. The transponders are wired to subsystem controllers that are in turn wired to a system controller. The central controller polls the cordless stations every 100 milliseconds to detect cordless station locations and to identify xe2x80x9cmissingxe2x80x9d cordless stations.
In U.S. Pat. No. 4,807,222, issued Feb. 21, 1989 N. Amitay described a LAN in which users communicate with RF or IR signals with an assigned Regional Bus Interface Unit (RBIU). Protocols such as CSMA/CD and slotted ALOHA are employed in communicating with the RBIUs.
In U.S. Pat. No. 4,402,090 issued Aug. 30, 1983, F. Gfeller et al. describe an infrared communication system that operates between a plurality of satellite stations and a plurality of terminal stations. A host computer communicates with the terminal stations via a cluster controller and the satellite stations, which may be ceiling mounted. Communication with the terminal stations is not interrupted even during movement of the terminal stations.
In IBM Technical Disclosure Bulletin, vol. 24, No 8, page 4043, January 1982 F. Gfeller describes general control principles of an infrared wireless network incorporating multiple ceiling mounted transponders that couple a host/controller to multiple terminal stations. Access to the uplink channel is controlled by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method.
What is not taught in the prior art is a method for reusing frequency hopping patterns thus allowing to operate such frequency hopping wireless LAN""s with a number of base stations greater than the number of existing frequency hopping patterns. More generally such method applies to the allocation of other kinds of network resources, such as a limited pool of remote stations addresses. In addition the method of the invention applies as well to wireless systems using different signalling techniques such as for instance direct sequence spread system radio frequency signalling or infrared and more generally to any communication system using a multiple access shared communication medium.
The invention as defined is to provide a method for reusing a limited number of network resources in a communication system using a multiple access shared communication medium such as a wireless radio frequency (RF) or infrared (IR) communication network comprised of a local area network connected to a plurality of base stations. Each base station has a geographic area, defined as a cell, within which remote stations are within reception range. Remote stations select one base station as home base station. Home base stations are capable of performing bidirectional communication with one or more remote stations under control of a controller connected to said local area network, the method of the invention comprises the steps of:
(a) requesting by a given base station assignment by said controller of one of said network resources; and
(b) selecting and assigning by said controller one of said network resources;
This method allows to select and assign one of the network resources already assigned to one or more other base stations. Selection is based on the computation by the controller of a distance index between the given base station and the other base stations. The resource assigned to the given base station is the one already assigned to one of the other base stations with the highest distance index to the given base station.
In a wireless communication system using frequency hopping RF communication, this method is particularly suited to reuse frequency hopping patterns when the number of active base stations exceeds the number of available frequency hopping patterns. Assigning a frequency hopping pattern already in use by a base station with the highest distance index reduces the risk of interference between two base stations using the same frequency hopping pattern.
Another aspect of the invention is to compute distance index based on data representative of base stations cells overlaps. Such cells overlap occurs at locations where a remote station is within RF or IR reception range of several active base stations. Cells overlaps information is used by the controller to reuse a network resource to assign it to a requesting base station. The method used to compute a distance index between the requesting base station and the other active base stations comprises the following steps:
(a) setting the distance index of the other base stations to an initial maximum value;
(b) setting the distance index to 1 for first level neighbor base stations, first level neighbors being defined as base stations whose cell overlaps with the requesting base station cell; and
(c) starting with n=1, performing a number of iterations consisting in setting the distance index to n+1 for (n+1) -th level neighbor base stations, (n+1) -th level neighbors being defined as base stations whose cell overlaps with the cell of a n-th level neighbor base station.